She’d done her research — extensive research. Ever since landing the role, she’d immersed herself in O.J. arcana: read Jeffrey Toobin’s book, “The Run of His Life,” on which the series is based, as well as Clark’s book, and prosecutor Chris Darden’s … studied the footage of the trial … watched all the news coverage. And she’d learned that Clark’s office chair was oversized; the petite prosecutor had been swallowed up in it. “I would have loved to have been able to have my body the way Darden described her,” she says wistfully over afternoon tea at L’Ermitage in Beverly Hills, where she offers a lecture on the benefits of half-and-half vs. nonfat milk — but opts for Splenda. (“Show me an actress who puts real sugar in her tea or her coffee, and I’ll show you an imposter,” she laughs.)

It was early in the filming, so she decided to keep silent about the chair. But as the seven-month shoot progressed, she would raise objections over any other inaccuracies she saw. She recalls one “prickly moment” with the head of wardrobe when she was presented with the wrong outfit for Clark’s closing arguments. “I just had the idea of Marcia sitting in her house watching it, saying ‘I didn’t wear that,’ ” Paulson says. That scrupulous attention to detail made her someone executive producer Ryan Murphy would come to rely on as much as the official on-set consultants during the production. “So many times I would start to ask a question, and Sarah knew it,” he recalls, “not just for Marcia, but for O.J. and for Shapiro.”

Early buzz on the series — and for Paulson’s performance — has been deafening. Count executive producers Nina Jacobson and Brad Simpson among her fans. “From the moment Ryan suggested her, I couldn’t think about anyone else,” Jacobson says. Adds Simpson, “I’ve never seen someone meld so completely with the character.”

But Paulson has tuned out the accolades — and everything else. In fact, she hasn’t even managed to bring herself to watch the show. The part is too important to her, and she’s worried she got it wrong. “There have been very few roles where I’ve finished, and gone into my trailer and just wept like a baby,” she says. “It happened after season two of ‘Horror Story’ and it happened after Marcia. I was so devastated to be finished doing it. If I could, it would be the part I would play for the rest of my life.”

“There have been very few roles where I’ve finished, and gone into my trailer and just wept like a baby. … It happened after Marcia.”

For Murphy, Paulson was the first — and only — choice for the part, even though other A-list stars (he won’t name names), wanted it. In fact, he didn’t give her a choice. “I thought she would knock it out of the park, and she did,” he says. “We built the cast around her.”

The director-star partnership between the two dates back to 2004, when he cast her in “Nip/Tuck” — but truly skyrocketed in the second, “Asylum,” season of “American Horror Story” with the plum role of journalist Lana Winters. Until then, she’d never been on a TV series — witness “Studio 60 on the Sunset Strip” — for longer than a season. She’s grateful to have handed the reins of her career over to him.

“I don’t know why I went from doing three episodes of ‘Horror Story’ as the psychic (in season one) to getting the part that was basically the story the next year,” she says. “I don’t know why that leap came. I’ve never been brave enough to ask him. It changed my life, completely. Now there’s nothing he could ask me to do that I would say no to. Ever.”

Murphy credits their working relationship to trust — and his respect for her work ethic on-set. “She cries if she thinks she doesn’t get it right,” he says. “She never rejoices in anything because she always thinks she can do it better. I don’t think I’ve ever done a scene with her where I’ve said, ‘Cut, I have it,’ where she didn’t say, ‘Let me do one more.’ ”

Paulson recalls a particularly challenging day on set, when Clark walks into the courtroom proud of her new haircut, and Judge Ito (Kenneth Choi) dismisses her with a snide “Welcome back, Ms. Clark, I think.” “I could feel the temperature rise in my neck, and I could feel the color in my face change,” Paulson says. “I was feeling Marcia’s humiliation and the indignity. I remember Ryan coming in after, and saying, ‘Do you want to go again?’ He looked at me and he said, ‘Let’s roll.’ ”

Adding to the emotional pressure was the challenge of juggling the simultaneous shoots of “Crime Story” and “Horror Story.” “It was very weird to be up and playing a junkie until 2 in the morning, and then have it be 9 a.m., and there I was, getting ready to do an opening statement or a cross-examination,” she says. “I think having played the twins the season before was a very, very good warm-up for playing the dual roles this year.”

CASE CLOSED: “I remember walking into the courtroom for the first time and feeling intimidated,” says Paulson, with Christian Clemenson as co-prosecutor Bill Hodgman. “I decided Marcia must have felt that, too.” Courtesy of FX

One thing is clear: The creatives’ relationship is built to last. “She’s not only a star, but a star who can act,” he says. “I’m never going to let her go.”

Though she’s renowned for her roles throughout the run of FX’s “American Horror Story” — including last season’s conjoined twins Bette and Dot Tattler — as well as the indelible Mistress Epps in “12 Years a Slave” — Paulson has played real people before, notably Sarah Palin aide Nicole Wallace in Jay Roach’s HBO movie “Game Change,” based on the 2008 presidential campaign. But “her voice wasn’t iconic, the way Sarah Palin’s was, and certainly not the way Clark’s was,” Paulson says. Roach gave her the freedom to play Wallace as she saw fit.

For “People v. O.J.,” the rules of engagement were different. “I know how much Marcia is identifiable, both in image and sound, because so many of us watched it,” she says. “There
was such an awareness to what she represented in that time in our country’s history.”

Ask people to describe Paulson, and one word comes up again and again: “chameleon.” “She’s the consummate actor,” says John Travolta, who plays Robert Shapiro. “We were in awe of her,” echoes Courtney B. Vance (Johnnie Cochran). “She’s a beast,” adds Sterling K. Brown (Chris Darden). She disappears into her roles, whether she’s playing a drug-addicted junkie in “American Horror Story: Hotel,” an abandoned lover in “Carol” or Marcia Clark — the embattled prosecutor we all think we know.

“I guess it’s a compliment,” she says with a laugh. “I don’t think anyone would have said, ‘You should play Marcia Clark; you look just like her.’ As an actress, I have range. My face doesn’t pigeon-hole me into any type of part.”

Ask her to describe herself, and the dominant word is “perfectionist” — hence, the obsessive attention to detail. “I know it sounds so actress-y, but I just wanted to do anything and everything I could to feel as connected to her,” she says. She painstakingly studied Clark’s mannerisms. She wore the perfume the prosecutor wore. She practiced pronouncing the word “de-fendant” as Clark did (emphasis on the first syllable), begging Murphy for retakes when she got it wrong. She had the props department make her a lighter with the words “truth and justice” engraved on it just like one Clark, a “big-time” smoker, carried. She even trained herself to write left-handed, as best as she could. Recalling a scene where Murphy wanted her to tap her pencil faster, she says with a laugh, “I could not become ambidextrous for this part.”

But when it came to meeting Clark herself, Murphy and the other producers discouraged it — for her and the rest of the cast, lest it influence their portrayal. Paulson calls it a smart decision. “I think I would have felt a desire to make sure that things were told in a particular way,” she allows.

It wasn’t until she’d finished taping episode six — aptly titled “Marcia, Marcia, Marcia” — that the actress, finally given the green light, reached out to Clark. “I wrote her a very long, very gushy email about how lucky I felt that I was getting to play her, how honored I was, how much I’d come to revere her, both in heart and mind,” she says.

And then she waited. “I must have checked my email every five minutes for about five hours,” she laughs. “It was like I was trying to ask someone out on a date and wanting to hear back from them. It was really embarrassing.”

As it turned out, Clark already had plans with a mutual friend the following night, and had assumed Paulson was joining them. It was kismet: It was a rare free night for Paulson, who was shooting both “Crime Story” and “Horror Story” simultaneously.

“I don’t think I’ve ever
done a scene with her where I’ve said, ‘Cut, I have it,’ where she didn’t say, ‘Let me do one more.'”Ryan MurphyBrinson+Banks for Variety

Their dinner at the Daily Grill in the San Fernando Valley began with tequila shots and lasted more than four hours, as they shut the restaurant down. “It was like meeting Emma Thompson or Meryl Streep or Gena Rowlands,” Paulson says.

It’s clear Paulson feels protective of Clark; she’s worried that this retelling is going to reopen long-healed wounds, as details get revealed that the public may have forgotten. The drama explores the rampant sexism that Clark endured, the humiliation of going through a divorce, her custody battles with her soon-to-be-ex husband. At the dinner, Paulson says she wanted to stress to Clark that the production wasn’t going to drag her name through the mud. “She didn’t have to worry that this was going to be another time where she was going to have to endure a public humiliation,” says the actress.

Those who took part in the trial were forever changed by it — especially Clark. A prosecutor who’d won 19 of 20 cases before the proceedings, she never went back into the courtroom in that role. Yet Paulson marvels at her ability to have rebuilt her life. “Marcia Clark today is a very successful, fully realized person,” she says. “I don’t know many people who could have undergone that kind of public scrutiny, be blamed for so much, and to continue to live a happy, engaged life.”

Adds Paulson, “My biggest hope is that it makes everybody rethink what they thought about her.”

Having wrapped the grueling task of the two intense roles, Paulson’s taking a break for now: Nothing’s officially on her docket just yet. She muses about doing a small, monthlong run of a play, or perhaps even a comedy. “Nobody thinks of me that way, but I do have a sense of humor. I want to be in the all-female ‘Ocean’s Eleven” with Sandra Bullock. Call me up, Sandy! Maybe one season of ‘American Horror Story’ before it’s over will be a pure farce.”

Much attention has been paid of late to her personal life: Paulson, 41, has been dating actress Holland Taylor, 73. “I don’t really understand the fascination with it,” she says “At the same time, I know it’s a little unconventional, so I get it to a degree.” She recalls being on vacation recently, and spotting a paparazzi drone flying in front of their house — she thought it was a model airplane. “I do feel like the more I talk about it, the more I feel like people are going to watch my work and not being able to see certain things that I’m playing because they think or know certain things about my life. That whole part of it makes me really uncomfortable.”

With buzz already building for the Emmys, Paulson, who’s been nominated multiple times yet never won, admits she wants that elusive trophy. “It never ceases to be an enormous honor, and I’ve had my heart broken from losing, from wanting it, and from being very disappointed when it didn’t happen. But it’s really a testament to how much I love what I do and how honored I am that I get to do it. I wanted to be an actress since I was in the womb. I make a living doing it, I get challenged by it, I’m inspired by it. So I’ve won. But the little girl in me would really love to clutch one of those and have my name on it.”

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